Poly(hydroxyalkanoic acids), also known as polyhydroxyalkanoates, have gained great attention in the plastics industry because they can be produced from renewable monomers. Because these polymers are biodegradable, they have been proposed for use in a broad range of applications. However, physical limitations such as brittleness and slow crystallization may limit the applications of poly(hydroxyalkanoic acids). Numerous impact modifiers have been developed in the past to improve the toughness of poly(hydroxyalkanoic acids). For example, US Patent Application No. 2006/0173133 discloses a toughened poly(hydroxyalkanoic acid) composition wherein an ethylene copolymer containing a comonomer with a glycidyl group (e.g., ethylene-butyl acrylate-glycidyl methacrylate (E/BA/GMA) terpolymer) is used as an impact modifier. However, due to safety concerns, the toughened poly(hydroxyalkanoic acid) compositions often need to undergo an additional wash step to remove the residual glycidal acrylates from the compositions. U.S. Patent Application Nos. 2006/0173133 and 2007/0255013 also disclose the addition of ethylene-alkyl acrylate dipolymers in poly(hydroxyalkanoic acid) compositions at 20 or at least 5 wt %. However, it has been found that the addition of the ethylene-alkyl acrylate dipolymer in such high levels often makes the poly(hydroxyalkanoic acid) too hazy to be useful.
Hence, it is desirable to develop an impact modifier that is free of residuals that might pose safety concerns so that the process for adding the impact modifier(s) is simplified and the resulting poly(hydroxyalkanoic acid) composition can be used as packaging materials directly. It is also desirable to develop an impact modifier that can be introduced into the poly(hydroxyalkanoic acid) composition at a level that maintains its clarity to an acceptable degree.